When the energy storable phosphor (e.g., stimulable phosphor, which gives off stimulated emission) is exposed to radiation such as X-rays, it absorbs and stores a portion of the radiation energy. The phosphor then emits stimulated emission according to the level of the stored energy when exposed to electromagnetic wave such as visible or infrared light (i.e., stimulating light). A radiation image recording and reproducing method utilizing the energy storable phosphor has been widely employed in practice. In that method, a radiation image storage panel, which is a sheet comprising the energy storable phosphor, is used. The method comprises the steps of: exposing the storage panel to radiation having passed through an object or having radiated from an object, so that radiation image of the object is temporarily recorded in the storage panel; sequentially scanning the storage panel with a stimulating light such as a laser beam to emit a stimulated light; and photoelectrically detecting the emitted light to obtain electric image signals. The storage panel thus processed is then subjected to a step for erasing radiation energy remaining therein, and then stored for the use in the next recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly used.
The radiation image storage panel (often referred to as energy storable phosphor sheet) used in the radiation image recording and reproducing method has a basic structure comprising a support and a phosphor layer provided thereon. However, if the phosphor layer is self-supporting, the support may be omitted. Further, a protective layer is generally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock.
Various kinds of phosphor layer are known and used. For example, a phosphor layer comprising a binder and an energy storable phosphor dispersed therein is generally used, and a phosphor layer comprising agglomerate of an energy storable phosphor without binder is also known. The latter layer can be formed by a gas phase-accumulation method or by a firing method. Further, still also known is a phosphor layer comprising energy storable phosphor agglomerate impregnated with a polymer material.
Japanese Patent Provisional Publication 2001-255610 discloses a variation of the radiation image recording and reproducing method. While an energy storable phosphor of the storage panel used in the conventional type plays both roles of radiation-absorbing function and energy storable function, those two functions are separated in the disclosed method. In the method, a radiation image storage panel comprising at least an energy storable phosphor (which stores radiation energy) is used in combination with a phosphor screen comprising another phosphor (radiation-absorbing phosphor) which absorbs radiation and emits ultraviolet or visible light. The disclosed method comprises the steps of: causing the radiation-absorbing phosphor of the screen or the panel to absorb and convert radiation having passed through an object or having radiated from an object into ultraviolet or visible light; causing the energy storable phosphor of the panel to store the energy of the converted light as radiation image information; sequentially scanning the panel with a stimulating light to emit stimulated light; and photoelectrically detecting the emitted light to obtain electric image signals.
The radiation image recording and reproducing method (or radiation image forming method) has various advantages as described above. However, it is still desired that the radiation image storage panel used in the method have as high sensitivity as possible and, at the same time, give a reproduced radiation image of high quality (in regard to sharpness and graininess).
In order to improve the sensitivity and the image quality, it is proposed that the phosphor layer of the storage panel be prepared by a gas phase-accumulation method such as vacuum vapor deposition, sputtering or chemical vapor deposition (CVD). The process of vacuum vapor deposition, for example, comprises the steps of: heating to vaporize an evaporation source comprising a phosphor or materials thereof by means of a resistance heater or an electron beam, and depositing and accumulating the vapor on a substrate such as a metal sheet to form a layer of the phosphor in the form of columnar crystals.
The phosphor layer formed by the gas phase-accumulation method contains no binder and consists of the phosphor only, and there are cracks among the columnar crystals of the phosphor. Because of the cracks, the stimulating light can stimulate the phosphor efficiently and the emitted light can be collected efficiently, too. Accordingly, a radiation image storage panel having that phosphor layer has high sensitivity. At the same time, since the cracks prevent the stimulating light from diffusing parallel to the phosphor layer, the storage panel can give a reproduced image of high sharpness.
As a process for reading out radiation image information from the storage panel, a line-scanning reading method is proposed so as to shorten the time of read-out, to downsize the apparatus and to reduce the cost. Japanese Patent Provisional Publication 2001-350230 discloses a radiation image information-reading apparatus for the line-scanning reading method. The disclosed apparatus comprises a line light source which irradiates the storage panel linearly with stimulating lights to cause stimulated emission, a stimulated emission-detecting means which receives and photoelectrically converts the stimulated emission given off by the panel from the area linearly exposed to the stimulating lights, a scanning means by which the storage panel and a combination of the light source and the detecting means are relatively moved in a direction (secondary direction of scanning) different from the longitudinal direction of the linearly exposed area (primary direction of scanning), and a reading means by which signals output from the detecting means are read in accordance with the movement. The detecting means comprises a linear light-receiving face whose width in the direction perpendicular to the longitudinal direction (that is, generally, a dimension of the face in the secondary direction of scanning) is designed so that 30 to 90% of the stimulated emission can be detected even though the emission is spread or diffused. Japanese Patent Provisional Publication 2001-350230 also discloses a graph showing a relationship between the diffusion of stimulated emission and the distribution of diffused emission intensity. The graph indicates that the storage panel processed in the above apparatus gives a stimulated emission of about 400 μm luminescence width (full width at half maximum, i.e., half-width).